1. Field of the Invention
The invention relates to the art of rendering electronic or digital images. The invention finds particular application in the rendering of text and line art images in color image reproduction systems.
2. Description of Related Art
Text and line art makeup an important class of printed and copied digital images. For example, this document, and the paperwork associated with it, is almost entirely made up of text and lines. The lines are used, for example, to separate sections of official forms.
Processing methods that would produce the most accurate and visually pleasing copies of text and line art are different than those used by most color image processing equipment. Existing transforms and image enhancement algorithms are optimized to produce accurate and pleasing spot color, business graphics, and photographic images.
Spot color applications require a high degree of color accuracy and purity. An example of a spot color application is the printing of a corporate logo. It is very important, when printing a corporate logo that colors be reproduced exactly, without any muddiness or colorant blending errors.
Business graphics applications include the printing of bar graphs and pie charts. In these applications, the user is less concerned about perfect color matching. Small differences, for example, between the colors in a graph as it was displayed on a computer screen (CRT) and as it is rendered by a printer are not very important. What is more important to the user is that the colors in the charts and graphs are crisp and uniform. Rendering methods or intents targeted for business graphics therefore, can and do make trade-offs. The goal of business graphics rendering intent is to produce vivid colors. To that end, perceived color matching and actual spectral content matching are sometimes sacrificed.
As the name implies, photographic applications include the reproduction of photographic scenes. The relationship between colors, and the sharpness of edges in a photographic image are usually considered more important than absolute color accuracy. Therefore, the photographic or perceptual intents make trade-offs that enhance color relationships and edge reproduction while sacrificing color accuracy.
An organization known as The International Color Consortium (ICC) has developed standards supporting color rendering intents called colorimetric, perceptual and saturation rendering intents. These rendering intents are best suited for rendering spot color, photographic images, and the pure hues of business graphics respectively. The ICC has not offered a rendering intent description optimized for text and line art.
Applying the available rendering intents to text and line art applications can produce undesirable results. For example, under certain circumstances, processing text and line art through the calorimetric, perceptual, or saturation intent can result in rendered images that contain distorting and displeasing halftone textures in the text or the background. One set of circumstances known to produce these results is the printing of small blue text on a gray background. The moiré patterns in the gray background combined with the halftone texture in the blue text results in reduced visibility and readability of the text. This quality reduction is unacceptable in most printing and copying applications.
Therefore there is a need for a processing method, or rendering intent, that is optimized for producing text and line art.